Acetic acid bacteria are microorganisms widely used in the vinegar production. Especially, acetic acid bacteria belonging to the genus Acetobacter or the genus Gluconacetobacter are utilized in the industrial acetic acid fermentation.
In the acetic acid fermentation, ethanol in the medium is oxidized and converted to acetic acid by acetic acid bacteria, and consequently, acetic acid is accumulated in the medium. However, since acetic acid is also inhibitory to acetic acid bacteria, the growth ability and acetic acid formation ability of acetic acid bacteria gradually decrease as the acetic acid concentration in the medium is raised due to the increase in the accumulation amount of acetic acid.
Accordingly, it has been desired to develop an acetic acid bacterium which is capable of fermenting acetic acid without decreasing the growth ability or acetic acid formation ability even at a higher acetic acid concentration, in other words, an acetic acid bacterium having a better acetic acid fermentation ability.
Many attempts are disclosed as an attempt to develop such acetic acid bacterium having an improved acetic acid fermentation ability. Examples of such attempts include: a transformant wherein three genes (aarA, aarB, aarC) contained in a gene cluster derived from an acetic acid bacterium are amplified, where these three genes are capable of restoring the acetic acid sensitive mutants of the acetic acid bacterium belonging to the genus Acetobacter to its original state (see for example, Patent document 1); an example in which a gene encoding a membrane-binding aldehyde dehydrogenase (ALDH) cloned from an acetic acid bacterium is introduced into an acetic acid bacterium (see for example, Patent document 2); and an example in which aconitase gene derived from an acetic acid bacterium is overexpressed (see for example, Patent document 3).
Actually, however, none of the attempts has succeeded in conferring a sufficient acetic acid fermentation ability. Therefore, it has been desired to obtain an acetic acid bacterium in which the acetic acid fermentation ability is enhanced and which is capable of more efficiently producing vinegar containing a high concentration of acetic acid.
On the other hand, the presence of an intercellular signal communication system in which transcription of specific genes is controlled depending on the cell density has been recently elucidated in many bacteria. This system is called quorum-sensing system (a control system sensing a cell density) and is involved in the expression control for various functions such as bioluminescence, exoenzyme production, toxic virulence, biofilm formation, and antibiotic production.
Two kinds of proteins are involved in the quorum-sensing system which has been found in many Gram negative bacteria such as Vibrio fischeri (see for example, Non-Patent document 1). The proteins are an acyl homoserine lactone synthase that synthesizes acyl homoserine lactone which is an intracellular signal molecule, and an acyl homoserine lactone receptor-type transcription factor that is a receptor of acyl homoserine lactone and that also functions as a transcription factor. Acyl homoserine lactone produced by an acyl homoserine lactone synthase in a bacterial cell diffuses inside and outside the bacterial cell. As the concentration of acyl homoserine lactone is increased, it forms a complex with the acyl homoserine lactone receptor-type transcription factor in a bacterial cell to control the gene transcription.
In spite that the quorum-sensing system has an important function as described above, no analysis on the quorum-sensing system in acetic acid bacteria has been carried out at all and its presence and function remained unknown. Further, it has not been conventionally known at all that the quorum-sensing system is involved in the acetic acid fermentation ability.    Patent document 1: Japanese Laid-Open Patent Application No. 3-219878    Patent document 2: Japanese Laid-Open Patent Application No. 2-2364    Patent document 1: Japanese Laid-Open Patent Application No. 2003-289867    Non-patent document 1: Bioscience and Industry, Vol. 60, No. 4, pp. 219-224, 2002